It is required that a toner binder used for a dry toner fulfills conflicting performances that the toner can be fixed even at a low hot-roll temperature (low-temperature fixing property) and the toner is not fused to a hot-roll even at a high hot-roll temperature (anti-hot offset property).
Conventionally, styrene-acrylic resin, polyester, epoxy resin and the like are used for toner binders, and a crosslinking polyester has been frequently used by virtue of being excellent in low-temperature fixing property.
In recent years, demanded for a toner binder and toner formed therefrom are a better low-temperature fixing property than before from the viewpoint of energy saving and a better anti-hot offset property from the viewpoint of downsizing of an apparatus such as copying machines and the like.
With a view to improving the low-temperature fixing property and anti-hot offset property of a toner binder of polyester, methods of mixing two polyesters having different molecular weight distributions have been proposed (for example, Japanese Patent Laid-Open No. 214368/1985, Japanese Patent Laid-Open No. 225244/1988, Japanese Patent Laid-Open No. 313760/1992 and soon), and the low-temperature fixing property and anti-hot offset property disclosed in these methods tend to be balanced better than those of conventional polyesters. However, toner binders in the prior art are formed by mixing two polyesters, which are not so much different in softening point, together. Meanwhile, in order to manufacture a toner binder having a better low-temperature fixing property and a better anti-hot offset property, it has been necessary to mix two polyesters, which are much different in softening point, together.
Also, the above-mentioned prior art involves the following problems separately.
More specifically, Japanese Patent Laid-Open No. 214368/1985 describes that a preferred mixing ratio of two polyesters (a, b) is such that (a) is at least 50 percent by weight and (b) is at most 30 percent by weight. Limitation in the mixing ratio has been inconvenient in achieving a better low-temperature fixing property of a toner binder. Japanese Patent Laid-Open No. 225244/1988 describes “It is preferable that the softening point Tsp of the second polyester is lower than a temperature which is 20° C. higher than the softening point Tsp of the first polyester”. The allowable range of a difference in softening point is disadvantageously too small to manufacture a toner binder having a better low-temperature fixing property and a better anti-hot offset property. Further, in the Laid-Open publication, the object of mixing two polyesters is directed to an improvement in pulverization property of a toner and self-crosslinkability of a toner adhered to a cleaning roller due to heat in addition to an improvement in low-temperature fixing property and anti-hot offset property. Therefore, nonlinear polyesters are selected for the first and second polyesters. Accordingly, the toner binders involve defects in transparency, and there has been room for improvement in the case of use for, in particular, color toner. Also, according to the disclosure of Japanese Patent Laid-Open No. 313760/1992, a toner binder is a mixture of polyesters and 20 parts of styrene-acrylic resin are added to 80 parts of toner binder at the time of manufacture of a toner. In some cases, toner with the styrene-acrylic resin added is inadequately decreased in lowest fixing temperature and a printed surface is poor in gloss.
Further, precise investigation has not been made in the prior art on the mixing condition of two polyesters. A toner binder formed by powder mixing polyesters, which are much different in softening point, together involves a problem that adequate dispersion of pigment cannot be made at the time of kneading of a toner. The pigment dispersibility is improved when a difference in softening point between two polyesters being subject to powder mixing is made small, but there is not attained the essential object of mixing two polyesters, directed to improvement in low-temperature fixing property and anti-hot offset property.
Hereupon, the first object of the invention is to provide a polyester toner binder, which is better in low-temperature fixing property and anti-hot offset property than that of the prior art.
The second object of the invention is to provide a toner binder having an excellent pigment dispersibility.
The third object of the invention is to provide a toner binder having other excellent qualities, which are generally required of a toner binder, such as stability of a toner, which is formed from the toner binder, in hot humid environment and cold, low and humid environment, heat storage stability, good charging property, and excellent glossiness of a printed surface if required.
Another object of the invention is to provide a method of manufacturing a polyester toner binder having excellent low-temperature fixing property, anti-hot offset property and pigment dispersibility.